APS News

A Century of Physics

1935-1945: Physics in World War II

By Hans Christian von Baeyer

Throughout the nineteen thirties, while America struggled with the Great Depression and Adolf Hitler's Nazis rose to absolute power in Germany, physicists quietly continued to collaborate across national boundaries. Quantum mechanics proved to be a reliable framework for the study of solid matter, of molecules, and of atoms. The discovery of the neutron and the invention of the cyclotron launched the new science of nuclear physics. Although the size of the nucleus is 100,000 times smaller than that of an atom, and its internal energy higher by the same amount, quantum mechanics continued to work perfectly. The future of physics looked promising. But by the end of the decade, World War II broke out in Europe and swept the whole world, including the physics community, into its wake. Physicists and engineers helped to win the air-borne Battle of Britain by developing Radar, and their German colleagues designed the V-2 rockets that terrorized London. Of greater historical significance, though, was the construction of the atomic bomb.

As soon as nuclear fission was discovered in Europe, it became apparent that if a way could be found to release its energy in a bomb, the course of the war would be altered. In America a number of physicists, many of European origin, worried that Hitler might acquire such a weapon and persuaded the normally pacifistic Albert Einstein to warn President Franklin D. Roosevelt. In an urgent letter dated August 2, 1939, he explained the danger by writing: "It is conceivable ... that extremely powerful bombs of a new type may thus be constructed." Einstein's letter did not have an immediate effect, but eventually helped to persuade the United States to begin the monumental task of building an atom bomb.

The man chosen to direct the project was the theoretical physicist Robert Oppenheimer. Although he had no industrial or even experimental experience, he proved to be a remarkably effective leader. His team on a remote mountain-top in New Mexico, and smaller groups in other secret laboratories, included most of the nation's best physicists. By the force of his towering intellect Oppenheimer managed to unite this fractious group in a common effort to design and build a bomb, and to test it successfully in July 1945. By then, Germany had already surrendered, but its ally Japan was still at war.

In August 1945, two atomic bombs dropped on the Japanese cities of Hiroshima and Nagasaki contributed to a quick end of World War II. Their chief legacy, however, was to be felt for a long time. For almost half a century the Cold War"s nuclear stand-off between the United States and the Soviet Union held the world in its grip.

Editor's Note: A CENTURY OF PHYSICS, a dramatic illustrated timeline wallchart of over a hundred entries on eleven large posters is intended for high schools and colleges. Each poster covers about a decade and is introduced by a thumbnail essay to provide a glimpse of the historical and scientific context of the time. In the August/September issue, APS News will feature 1945-1954 The Post-War Boom.


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Editor: Barrett H. Ripin